networkn:richms: Antenna being going to a home theater reciever, TV, etc that is connected to the same computer or DAC somehow, also USB cables carry a chassis ground so unplug any of those going off to external drives and printers etc that are powered seperatly.
Ok. I did all that already in the troubleshooting in my OP. I don't have any gear not listed, connected (Unless you are talking indirectly as in, on the same electrical circuit in the house).
With absolutly no input filtering or visible filtering on the AC from the transformer other than some large electorlytics which are useless for high frequancy noise, there is the real chance that you are just picking up RF noise which the circuitry is demodulating, and if it comes from something else on the earth wire from something else having a noisy switching power supply then it will come across as a buzz at mains frequancy normally.
The construction looks like a bad noisy 60 year old valve radio, so I wouldnt be expecting much more than that qualitywise from it.